Mine looks like this:
unix:path=/run/user/1000/bus
I’m guessing that you have something spawning dbus instead of having it socket activated and /tmp is then a separate filesystem inside the env.
Set services.dbus.socketActivated = true; (this really should be the default), nixos-rebuild, log out and back in, check $DBUS_SESSION_BUS_ADDRESS and if it looks like mine, it should work.